The present relates to a gate turn-off thyristor (GTO) module. More particularly, the invention relates to the arrangement of outer electrodes in a package of a GTO module.
At present, much effort is underway to devise new techniques to conserve energy. Particularly, a great deal of effort has gone into saving electrical energy through the provision of improved high-power switching devices. The GTO is one device which has shown much promise because of its small physical size but large current switching ability. This device finds use, for instance, in inverter circuits used for motor control.
It has been the practice to modularize circuits employing GTOs in order to reduce their overall size. FIG. 1 shows an example of a GTO module of the prior art. In this module, two GTO chips, GTO1 and GTO2, are connected in series. A portion 100 encircled by a broken line represents an internal part of package. Reference numeral 101 designates an output anode terminal A2 connected to the anode electrode of GTO2, 102 a main cathode terminal K1 connected to the cathode electrode of GTO1, and 103 a common main terminal connected to the anode electrode of GTO1 and to the cathode electrode of GTO2. The terminal 103 also serves as an anode terminal A1 of GTO1 and as a cathode terminal K2 of GTO2. 108 and 109 indicate a control gate terminal G1 and a cathode terminal K1a connected, respectively, to the gate electrode and the cathode electrode of GTO1. 110 and 111 designate a control gate terminal G2 and a cathode terminal K2a connected, respectively, to the gate electrode and the cathode electrode of GTO2.
Generally, in the case where the GTO is employed in an inverter or chopper ciruit, a snubber circuit is connected between the anode electrode and the cathode electrode of the GTO. One of the reasons for using the snubber circuit is to bypass the current flowing through the GTO at the time of turning off the GTO to decrease swiftly the current flowing through GTO, thereby to reduce the power loss in the GTO. Another reason is to prevent re-ignition at the turn-off failure of the GTO by suppressing spike voltage build-up by inductance in the snubber circuit during fall period.
The snubber circuit is connected between anode and cathode terminals of each of the two GTOs (GTO1 and GTO2) in a two-device module. More specifically, in FIG. 1 reference numeral 140 indicates a snubber circuit for GTO1 composed of a diode D.sub.s1 and a capacitor C.sub.s1 connected in series, and a resistor R.sub.s1 connected in parallel across the diode D.sub.s1. 150 indicates a similar circuit for GTO2 composed of a diode D.sub.s2, a capacitor C.sub.s2, and a resistor R.sub.s2. The values of the capacitors and resistors and the types of diodes forming the snubber circuits are selected to provide optimum characteristics of the GTO module in accordance with the particular application at hand.
FIG. 2 is a perspective view showing the exterior configuration of a conventional GTO module having an internal structure as shown in FIG. 1. In FIG. 2, 100 indicates a resin package and 130 a heat radiating plate made of copper sheeting. 101, 102 and 103 designate the main electrode terminals, 108 and 109 control electrode terminals of GTO1 and 110 and 111 control electrode terminals of GTO2. 120 indicates a hole used to fix the module to a radiating fin by means of a screw.
FIG. 3A shows such a GTO module applied to an inverter circuit. Specifically, FIG. 3A shows one phase of the inverter circuit. FIGS. 3B through 3E show voltage and current waveforms at respective points taken at the turn-off time of the GTO devices. Specifically, FIG. 3B shows the waveform of a voltage V.sub.AK across the anode and cathode of a GTO at turn-off. As seen in FIG. 3B, voltage transients indicated by .DELTA.V.sub.AK and V.sub.DP occur during the turn-off time. These can be expressed by the following equations: ##EQU1## where, L.sub.T1 =l.sub.3 +l.sub.4 +l.sub.5 +l.sub.6 +l.sub.8 +, L.sub.T2 =l.sub.1 +l.sub.2 +l.sub.3 +l.sub.4, and l.sub.1, l.sub.2, l.sub.3, l.sub.4, l.sub.5, l.sub.6, l.sub.7, and l.sub.8 are inductances of respective connecting wires as indicated in FIG. 3A. Among the transients, .DELTA.V.sub.AK is important to the breakdown voltage of the GTO and V.sub.DP is important with respect to the switching loss in the GTO at the time of turn-off. The voltage V.sub.DP increases in proportion to the wire inductance L.sub.T2. Also, as the wire inductance L.sub.T2 increases, the switching loss at turn-off increases. The GTO may in fact be destroyed if the switching loss is sufficiently high. Accordingly, it is necessary to shorten the lengths of the connecting wires to, from and within the snubber circuit to reduce the inductances of those wires.
In a conventional GTO module having a wire structure and exterior configuration as shown in FIGS. 1 and 2, the snubber circuit 140 for GTO1 is connected between the main anode terminal 103 and main cathode terminal 102 and the snubber circuit 150 for GTO2 is connected between main anode terminal 101 and main cathode terminal 103. Such a GTO module has the following drawbacks:
(1) Because the main electrode terminals serve commonly as connection terminals for the snubber circuit, the total length of the wiring of the snubber circuit and the wiring to the electrode main terminals of the GTO is determined principally by the size of the device package, and hence even if the external wiring of the snubber circuit were shortened, the total inductances would not be sufficiently reduced.
(2) Because the main electrode terminals serve commonly as connection terminals to connect the snubber circuit to the GTO, it is necessary to connect two wire to each main electrode terminal, hence making the apparatus difficult to assemble and repair.